This invention relates to a method of examining quality of cerium-based abrasives, a method of producing a cerium-based abrasive, and a cerium-based abrasive.
Recently, glass materials have been used for various purposes. Of these, advanced glass materials including glass substrates for optical and magnetic disks, active matrix type liquid crystal displays (LCDs), color filters for liquid crystal TV sets, watches, calculators, LCDs for cameras, displays for solar cells or the like, LSI photomasks and optical lenses, and optical lenses themselves are required to be surface-ground highly precisely.
These glass substrates are normally surface-ground using a cerium-based abrasive composed of an oxide of rare-earth, in particular cerium oxide, as a main ingredient, because cerium oxide shows several times higher grinding efficiency than zirconium oxide and silicon dioxide for grinding glass materials.
The common stock materials for cerium-based abrasives include rare-earth materials, e.g., carbonates, hydroxides and oxalates of rare-earth elements, and oxides produced by burning them. These stock materials are normally prepared from bastnasite concentrate or other cerium-containing rare-earth materials by removing a part of rare-earth elements, e.g., neodymium (Nd) and prasceodymium (Pr), and radioactive materials and the like by a known chemical treatment.
A cerium-based abrasive from a carbonate or oxide of rare-earth is produced by the following process. The process starts with slurrying or wet-crushing the stock material followed by chemical treatment with a mineral acid and, as required, with hydrofluoric acid or ammonium fluoride. The resultant slurry is subjected to filtration, drying and roasting. Finally, it is crushed and classified to have an abrasive of specific particle size.
A cerium-based abrasive is required to have specific grinding characteristics for specific purposes. It is therefore necessary to grasp the grinding characteristics of a produced abrasive by, e.g., analyzing these characteristics. For example, it is known that grinding speed, referred to as grindability as one of important grinding characteristics, increases as the abrasive crystal size grows by the roasting step. It is however difficult to accurately grasp the grinding characteristics only from average particle size determined by the common method of analyzing particle size distributions. Therefore, an abrasive is examined, as required, for its quality by a test in which a test piece is actually ground by the abrasive. The grinding test is time-consuming, because the ground test piece is weighed to determine grindability, or ground surface is observed to confirm scratches. Therefore, a more simple method of examining the quality has been in demand. At the same time, a method of producing abrasives with specific grinding characteristics can greatly save works for, e.g., quality examination, and hence is desirable, because it produces abrasives more efficiently.
The present invention is developed in the light of the above problems. It is an object of the present invention to provide a method of examining quality of cerium-based abrasives which can simply determine their grinding characteristics. It is another object of the present invention to provide a method of producing a cerium-based abrasive which can give cerium-based abrasives with specific grinding characteristics. It is still another object of the present invention to provide a cerium-based abrasive which has specific grinding characteristics for specific purposes.